1. Field of the Invention
The present invention relates to optoelectronic devices, in particular but not exclusively to the integrated combination of a laser amplifier and an optical detector, and to a method of making such devices.
Such devices are of use, for example, in optical communications.
2. Related Art
As the demand for telecommunications services increases, the demand for high-capacity transmission systems also increases. To meet this demand the large bandwidth of optical fibres will need to be used much more efficiently in the future than in present optical communications systems.
Currently one of the principal limitations in the development of high-bandwidth optical communications systems is the availability of sensitive wideband optical receivers.
Present receiver designs tend to have either good sensitivity or wide bandwidth, but not both. Wideband receivers based on PIN photodiodes suffer from low sensitivity, whereas those based on avalanche photodiodes (APDs) are limited by gain-bandwidth product.
One approach to overcoming these limitations is to use a PIN photodiode receiver but to improve the receiver's sensitivity by preamplifying the incoming signals using a laser amplifier, as described in the paper by I. W. Marshall and M. J. O'Mahony, Electronics Letters, Vol. 23, pp 1052-1053, 1987. In this arrangement, a travelling wave semiconductor laser amplifier (TWSLA) and a PIN photodiode are packaged in hybrid form with intermediate collection and focussing optics, which optics may include a narrowband optical filter for noise reduction.
While the performance of that receiver was commendable--an equivalent input noise of 1 p A Hz.sup.-1/2 with a 3 dB bandwidth of 10 GHz, and a sensitivity at 15 GBit/s of -27 dBm, it would be desirable if similar or better performance could be achieved without the need for focussing optics and hybrid construction.
In theory it would be a trivial matter to combine a laser amplifier and a photodetector in a single integrated component. Indeed, such structures already exist, albeit that the laser component is optimised for use as a resonant device rather than as an amplifier.
Maturer reflection suggests, however, that perhaps the production of a high performance integrated component is not quite so simple!
The laser/detector combinations which are known in the art have all been produced essentially as laser sources with integrated, and hence pre-aligned, back-facet detectors. Such detectors, which were used for laser control purposes, do not need to have any high speed capabilities. Even so, it was found that, in general, the performance of the laser was compromised by the need to produce a detector in the same process. Consequently, such laser/detector combinations, while common in the late 1960s and through the 1970s, are no longer in favour. Nowadays with the higher demands on laser performance, it is apparently deemed not practical or not worthwhile producing integrated laser devices. Moreover, in general the added processing complexity occasioned by the production of two device types in a single processing sequence leads to an unacceptable reduction in yield.
It is believed that many research groups around the world have tried to produce an integrated laser amplifier/PIN, but have found the problem too intractable. Meanwhile of course, others have been working on improving the performance of APDs, discrete PIN photodetectors, and discrete laser amplifiers. A further alternative to the problem of low receiver sensitivity is to use semiconductor laser amplifiers or fibre-laser amplifiers, such as erbium amplifiers, in the fibre link to increase the signal level reaching the receiver.
Very surprisingly, despite the seeming intractability of the problem, we have devised an integrated laser amplifier/PIN which not only offers very high levels of performance, but is also very simple to fabricate.
According to a first aspect the present invention provides an optoelectronic semiconductor device comprising a first epitaxial layer grown on a semiconductor substrate, a second epitaxial layer grown on said first layer and having a higher refractive index than that of the first layer and a third epitaxial layer in the form of a ridge structure selectively grown on or over said second layer, wherein said first epitaxial layer is of a first conductivity type, said third epitaxial layer is of a second conductivity type and a p-n junction is formed in said second epitaxial layer between a region of material of said second conductivity type, which is aligned beneath said ridge, and adjacent regions of said first conductivity type of said second layer.